Multilayered polyethylene material and ballistic resistant articles manufactured therefrom

ABSTRACT

The present invention relates to polyethylene material that has a plurality of unidirectionally oriented polyethylene monolayers cross-plied and compressed at an angle to one another, each polyethylene monolayer composed of ultra high molecular weight polyethylene and essentially devoid of resins. The present invention further relates to ballistic resistant articles that include or incorporate the inventive polyethylene material and to methods of preparing the material and articles incorporating same.

This application is a continuation of commonly owned co-pending U.S.application Ser. No. 11/204,847, filed Aug. 15, 2005, which claims thebenefit of U.S. Provisional Application Nos. 60/601,645, filed Aug. 16,2004 and 60/626,206, filed Nov. 8, 2004, the entire contents of each ofwhich are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a novel polyethylene materialcomprising a plurality of unidirectionally oriented polyethylenemonolayers cross-plied and compressed at an angle to one another, eachpolyethylene monolayer formed of ultra high molecular weightpolyethylene and essentially devoid of resins. The present inventionfurther relates to ballistic resistant articles comprising thepolyethylene material and methods of preparing same.

BACKGROUND OF THE INVENTION

Ultra high molecular weight (UHMW) polyethylene is an ethylene polymerwith an extremely high molecular weight of one million or greatercharacterized by high resistance impact.

Ballistic resistant polymer monolayers, including inter aliapolyethylene monolayers, are typically formed from fibers, a solution ora powder of the polymer. Polymer fibers are woven, knitted or not wovenand monolayers composed from these fibers typically comprise an elasticresin or a polymeric matrix that encapsulate and holds the fiberstogether (see example, U.S. Pat. Nos. 4,574,105; 4,820,568 and 4,944,974among others).

The prior art teaches that the percentabe of resin, bonding materialsand the like must not exceed 205 of the total weight of a ballisticresistant material, otherwise the anti-ballistic qualities of thematerial begin to deteriorate. For example, European Patent No. 768,507to van de Goot et al, discloses a ballistic-resistant article containinga compressed stack of monolayers containing unidirectionally orientedreinforcing aramid fibers and a matrix consisting of a polymer, thecontent of which is at most 25 weight percentage, the fiber direction ineach monolayer being rotated with respect to the fiber direction in anadjacent monolayer. Ballistic resistant articles comprising successivelayer of fibers in a matrix composition, the matrix composition is about20 weight percentage of the total weight are disclosed in U.S. Pat. Nos.5,552,208 and 5,587,230 among others.

U.S. Pat. No. 5,340,633 to van der Loo discloses a multilayeredantiballistic structure comprising a first layer which comprises ceramictiles, a second layer of composite material comprising polyalkenefilaments, a matrix that surrounds the polyalkene filaments and anintermediate layer of a material between the first and the secondlayers, having a flexural modulus which is higher than that of thematerial of the second layer and lower than that of the ceramicmaterial.

A ballistic resistant article constructed of high performance fibers anddevoid of resins is disclosed in U.S. Pat. No. 5,935,678 to Park. Thearticle includes two arrays of high performance,unidirectionally-oriented fiber bundles, cross-plied at an angle withrespect to one another in the absence of adhesives or bonding agents.Thermoplastic films, including inter alia polyethylene films, are bondedto the outer surfaces of the arrays without penetration of the filmsinto the fiber bundles. This arrangement substantially reduces theweight of the resulting article without compromising the anti-ballisticcharacteristics thereof.

Formation of UHMW polyethylene tapes and films from a powder ofpolyethylene rather than from polyethylene fibers is known in the art.U.S. Pat. No. 4,879,076 to Kobayashi et al. discloses a process forproducing a polyethylene material of great mechanical strength and highelastic modulus from particulate UHMW polyethylene, wherein theparticulate UHMW polyethylene is obtained from polymerization ofethylene at a temperature between 20° C. to 110° C. in the presence of acatalyst comprising magnesium, vanadium and an organometallic compound.

U.S. Pat. No. 5,091,133 to Kobayashi et al. discloses an improvement tothe process disclosed in the U.S. Pat. No. 4,879,076. The process isdirected to continuous production of a high-strength and high moduluspolyolefin material, comprising feeding a polyolefin powder between acombination of endless belts, compressing-molding the polyolefin powderin a temperature below the melting point thereof and rolling theresultant compression-molded polyolefin followed by stretching.

U.S. Pat. No. 5,106,555 to Kobayashi et al. discloses anotherimprovement to the processes disclosed in the U.S. Pat. Nos. 4,879,076and 5,091,133, the improvement including: (i) using an UHMW polyethylenepowder that has an intrinsic viscosity of 5-50 dl/g as measured at 135°C. in decalin as a principal component in the process and (ii)concurrently processing, in at least one of the compression-molding stepand rolling step, an olefin polymer having an intrinsic viscosity from0.5-3 dl/g.

Another improvement to the processes for continuous production of ahigh-strength and high modulus polyolefin material is disclosed in U.S.Pat. No. 5,106,558 to Kobayashi et al. The process comprises, prior tofeeding and compressing-molding the polyethylene powder, the step ofmixing an UHMW polyethylene powder with a liquid organic compound havinga boiling point higher than the melting point of said polyethylene.

U.S. Pat. No. 5,200,129 to Kobayashi et al. discloses an alternativeprocess for continuously producing a high-strength and high-moduluspolyolefin material. The process comprises using, at thecompression-molding step, particular pressing means comprising twoopposing sets of a plurality of rollers, the rollers in each set are notconnected together as disclosed in the U.S. Pat. No. 5,091,133, rathereach roller is rotatably supported at the shaft ends by a frame.

A split polyethylene stretched tape produced by subjecting UHMWpolyethylene to compression-molding, rolling, stretching and then tosplitting is disclosed in U.S. Pat. No. 5,578,373 to Kobayashi et al.The split polyethylene has a tensile strength of 0.7 to 5 GPa whentwisted in the range of 50-500 times/m.

Despite the foregoing materials and processes, there still remains aneed for polymeric materials having enhanced properties for use inantiballistic articles, and these are now provided by the presentinvention.

SUMMARY OF THE INVENTION

The present invention relates to a multilayered polyethylene materialessentially devoid of resins, adhesives and bonding agents, the materialcomprising a plurality of unidirectionally oriented polyethylenemonolayers cross-plied at an angle with respect to one another.According to certain embodiments each unidirectional polyethylenemonolayer comprises at least two unidirectional polyethylene strips, thepolyethylene strips formed by compression-molding and stretching a solidform of ultra high molecular weight polyethylene, substantially devoidof resins or adhesives.

The known art does not disclose or teach a material comprisingunidirectional monolayers consisting essentially of ultra high molecularweight polyethylene and essentially devoid of resins, where themonolayers are cross-plied and compressed at an angle to one another.The inventors have found that this material exhibits antiballisticproperties, improved tenacity, exceptional rigidity and capability ofmaintaining its form when exposed to ballistic impact, thus rendering iteminently suitable for use in antiballistic or ballistic resistantarticles. The invention also relates to an article comprising thematerial described herein in combination with an additional materialselected from the group consisting of: ceramic, steel, aluminum,titanium, glass and graphite. Advantageously, the article is anantiballistic or a ballistic resistant article in the form or a fabricenvelope or supporting structure, preferably one that can be worn, i.e.,a garment.

This invention further relates to methods of preparing unidirectionalmonolayers of the type mentioned herein as well as to the preparation ofbilayered materials that incorporate or include the unidirectionalmonolayers therein. Methods of making antiballistic or ballisticresistant articles that include the steps of these methods are alsoprovided.

Other features and advantages of the present invention will become clearfrom the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the invention and the accompanying drawings in which:

FIG. 1 demonstrates a role of a polyethylene strip.

FIG. 2 presents a polyethylene monolayer comprising four overlappingpolyethylene strips.

FIG. 3 exhibits a plurality of polyethylene strips laid at an angle ofsubstantially 90 degrees over a unidirectional UHMW polyethylenemonolayer.

FIG. 4 demonstrates a polyethylene article comprising a plurality ofpolyethylene monolayers cross-plied at an angle of substantially 90degrees to one another and compressed at about 120° C. under a pressureof about 140 bar.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a ballistic resistant articlepolyethylene consisting essentially of ultrahigh molecular weightpolyethylene, the article comprising a plurality of unidirectionallyoriented polyethylene monolayers cross-plied, positioned and arranged atan angle with respect to one another and attached to each other in theabsence of any resin, bonding matrices and the like.

The ultra high molecular weight polyethylene material of the inventionis exceptionally rigid, highly impenetrable and particularly light. Forexample, a bilayered material consisting of two monolayers will have atypical areal density of about 125 g/m² and an average thickness of 100to 160 μm though it is to be understood explicitly that these values aremerely representative values.

The present invention further provides a ballistic resistant articledevoid of resins, adhesives and bonding agents, the article comprising aplurality of layers of ultrahigh molecular weight polyethylene materialaccording to the principles of the invention and having any desiredthickness, shape or contour.

The present invention is based in part on the unexpected discovery thata polyethylene article, consisting essentially of ultra high molecularweight polyethylene and essentially devoid of resins and other bondingmaterials, exhibits improved antiballistic and tenacity properties,exceptional rigidity at an outstanding low weight per area as comparedto ballistic resistance materials known in the art.

The polyethylene material and article of the present invention areparticularly advantageous over previously known antiballistic materialsas they provide the following features:

-   -   1. Low weight—the ballistic resistant material of the present        invention afford the manufacturing of ballistic resistant molded        articles that provide at least the same level of protection as        the known molded articles at a significantly lower weight. Low        weight per unit area is of great importance in many        applications. This is the case, for instance, in the field of        personal protective equipment such as helmets, shields, shoes        and the like. Low weight is also essential for the application        of ballistic-resistant molded articles in for instance        helicopters, motorcars and high-speed, highly maneuverable        combat vehicles;    -   2. Economical—the starting material (essentially UHMW        polyethylene) is inexpensive and the manufacturing process is        relatively short and thus cost effective, as compared with        antiballistic materials and processes of making same as are        known in the art;    -   3. Improved heat stability—due to the absence of resins and        other bonding materials the material of the invention is stable        at high temperatures, particularly at the temperatures resulting        from a ballistic impact or the operating temperatures typically        associated with a variety of engines such as internal combustion        engines. Additionally, the material of the invention and        articles consisting therefrom have exceptionally long shelf-life        due to their heat stability;    -   4. Deformation resistance—due to the absence of resins and other        bonding materials and due to the low elasticity of the UHMW        polyethylene monolayers the article maintains its structure even        under an extreme impact, such as a ballistic impact; and    -   5. Can be easily laminated to other materials—ballistic        resistant articles with large surface areas and any desired        shape and curvature can be readily produced from the        polyethylene material of the invention. Such articles may be        applied as a protective cover to other materials and articles.

According to one aspect, the present invention provides a materialcomprising a plurality of unidirectional monolayers consistingessentially of UHMW polyethylene and essentially devoid of bondingmatrices, with the direction of each monolayer being rotated at an anglewith respect to the direction in an adjacent unidirectional monolayer.

According to one embodiment, the material comprising two monolayerscompressed at an angle to one another.

According to certain embodiments, the direction of each unidirectionalmonolayer is rotated or positioned with respect to the direction in anadjacent unidirectional monolayer at an angle of 20 to 160 degrees.According to other embodiments, the angle is 70 to 110 degrees.According to yet other embodiments, the angle is substantially 90degrees. Of course, the term “angle” does not include angles of 0, 180or multiples of 180 as those arrangements would have the directions ofthe monolayers in parallel alignment.

According to yet another embodiment, the material comprises a pluralityof unidirectional monolayers wherein each monolayer has a tensilestrength of 8-40 cN/dTex, preferably 10 to 35 cN/dTex. One cN/dTex (orcN/dtex) is equivalent to one centi-Newton per one deciTex (i.e., 10⁻¹of a Tex) wherein a Tex is defined as 1·10⁻⁶ kg/m.

According to yet another embodiment, the material comprises a pluralityof unidirectional monolayers wherein each unidirectionally orientedmonolayer is 30-120 μm thick. According to yet another embodiment, eachunidirectionally oriented monolayer is 50-100 μm thick.

According to yet another embodiment, the material comprises a pluralityof unidirectional monolayers wherein the areal density of eachunidirectionally oriented monolayer is within the range of 60 to 200g/m².

According to yet another embodiment, the material comprises a pluralityof unidirectional monolayers wherein each unidirectionally orientedmonolayer has an elastic modulus of 400 to 1,200 cN/dTex. According toyet another embodiment, each unidirectionally oriented monolayer has anelongation modulus of 2-6%. According to yet another embodiment, eachunidirectionally oriented monolayer has a maximal displacement of 2-12mm per 200 mm.

According to yet another embodiment, the material comprising a pluralityof unidirectional monolayers wherein each unidirectionally orientedmonolayer consists essentially of ultra high molecular weightpolyethylene.

According to various embodiments, the material is used in combinationwith an additional material selected from the group consisting of:ceramic, steel, aluminum, titanium, glass and graphite.

According to yet another embodiment, the material is a ballisticresistant article. In some embodiments, the ballistic resistant articlecomprises between 70 to 280 monolayers.

According to yet another embodiment, the ballistic resistant article iscontained within a fabric envelope. According to yet anotherembodiments, the ballistic resistant article is contained within asupporting structure for being worn on a body part. According to yetanother embodiment, the ballistic resistant polyethylene article iscontained within a body garment.

According to yet another aspect, the present invention provides a methodfor the continuous preparation of a unidirectional monolayer consistingessentially of ultrahigh-molecular-weight polyethylene, comprising:

-   -   (a) providing a plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips;    -   (b) aligning the plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips in the same        direction, wherein adjacent strips partially overlap; and    -   (c) compressing said plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips thereby obtaining        a unidirectional ultrahigh-molecular-weight polyethylene        monolayer.

According to one embodiment, compressing the plurality of unidirectionalultrahigh-molecular-weight polyethylene strips according to the methodof this aspect occurs at a temperature below the melting point of theultrahigh-molecular-weight polyethylene. According to anotherembodiment, compression temperature is within the range of 110 to 150°C.

According to another embodiment, compressing said plurality ofunidirectional ultrahigh-molecular-weight polyethylene strips occurs ata temperature below the melting point of the ultrahigh-molecular-weightpolyethylene and under a pressure of 10 to 100 N/cm².

According to yet another embodiment, the unidirectionalultrahigh-molecular-weight polyethylene monolayer obtained by the methodof this aspect is essentially devoid of bonding matrices.

According to yet another embodiment, the unidirectionalultrahigh-molecular-weight polyethylene monolayer obtained by the methodof this aspect is 30-120 μm thick, alternatively 50-100 μm thick.According to yet another embodiment, the areal density of theunidirectional ultrahigh-molecular-weight polyethylene monolayer iswithin the range of 60-200 g/m².

According to yet another embodiment, the unidirectionalultrahigh-molecular-weight polyethylene monolayer obtained by the methodof this aspect has a tensile strength of 10 to 25 cN/dTex. According toyet another embodiment, the unidirectional ultrahigh-molecular-weightpolyethylene monolayer has an elastic modulus of 400 to 1,200 cN/dTex.According to yet another embodiment, the unidirectionalultrahigh-molecular-weight polyethylene monolayer has an elongationmodulus of 2-6%. According to yet another embodiment, the unidirectionalultrahigh-molecular-weight polyethylene monolayer has a maximaldisplacement of 2-12 mm per 200 mm.

According to an alternative embodiment, the method for the continuouspreparation of unidirectional monolayers further comprising after step(b) and before step (c) the step of:

-   -   laying a polymeric film having a melting point substantially        lower than the melting point of the polyethylene strips over        each area of contact between two adjacent unidirectional        ultrahigh-molecular-weight polyethylene strips, the polymeric        film is laid in parallel to the direction of the aligned        plurality of unidirectional ultrahigh-molecular-weight        polyethylene strips.

According to one embodiment, the polymeric film is selected from thegroup consisting of low density polyethylene, high density polyethylene,polyamide, polypropylene, polyester, polystyrene, polyethylene,polycarbonate and poly(methyl methacrylate). According to oneembodiment, the polymeric film is 4 to 25 μm thick.

According to yet another aspect, the present invention provides a methodfor the continuous preparation of a material comprising a plurality ofunidirectional monolayers consisting essentially ofultrahigh-molecular-weight polyethylene and essentially devoid ofbonding matrices, wherein the direction of each monolayer being rotatedwith respect to the direction of an adjacent unidirectional monolayer,the method comprising:

-   -   (a) providing a first unidirectional ultrahigh-molecular-weight        polyethylene monolayer;    -   (b) providing a plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips;    -   (c) aligning the plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips such that each        strip is oriented in parallel to adjacent strips, wherein        adjacent strips partially overlap, said plurality of strips is        laid over the first unidirectional ultrahigh-molecular-weight        polyethylene monolayer, wherein the direction of said plurality        of strips being rotated with respect to the direction of said        first unidirectional ultrahigh-molecular-weight polyethylene        monolayer at an angle; and    -   (d) compressing said plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips laid over said        unidirectional ultrahigh-molecular-weight polyethylene        monolayer, thereby obtaining a bilayered material consisting        essentially of ultrahigh-molecular-weight polyethylene and        essentially devoid of bonding matrices, comprising two        compressed unidirectional monolayers with the direction of each        monolayer being rotated at an angle with respect to the        direction in an adjacent unidirectional monolayer.

According to certain embodiments, the direction of said plurality ofstrips provided in the method of this aspect, is rotated at an angle of20 to 160 degrees with respect to the direction of the firstunidirectional ultrahigh-molecular-weight polyethylene monolayer.According to other embodiments, the angle is 70 to 110 degrees.According to yet other embodiments, the angle is substantially 90degrees.

According to an alternative embodiment, the method for the continuouspreparation of a material comprising a plurality of unidirectionalmonolayers further comprises after step (c) and before step (d) the stepof laying a polymeric film having a melting point substantially lowerthan the melting point of the polyethylene strips over each area ofcontact between two adjacent unidirectional ultrahigh-molecular-weightpolyethylene strips, the polymeric film being laid in parallel to thedirection of the aligned plurality of unidirectionalultrahigh-molecular-weight polyethylene strips.

According to one embodiment, the polymeric film is 4 to 25 μm thick.According to another embodiment, the polymeric film is selected from thegroup consisting of: low density polyethylene, high densitypolyethylene, polyamide, polypropylene, polyester, polystyrene,polyethylene, polycarbonate and poly(methyl methacrylate).

According to another embodiment, step (d) of the method for thecontinuous preparation of a material comprising a plurality ofunidirectional monolayers occurs at a temperature below the meltingpoint of the ultrahigh-molecular-weight polyethylene. According to yetanother embodiment, step (d) occurs at a temperature between 100° C. to150° C. According to yet another embodiment, step (d) occurs at atemperature below the melting point of the ultrahigh-molecular-weightpolyethylene and under a pressure of 10 to 100 N/cm².

According to one embodiment, the bilayered material obtained by themethod of this aspect has a tensile strength within the range of 8 to 40cN/dTex. The tensile strength is obtained irrespective of the directionof the strips in each monolayer. According to yet another embodiment,the bilayered material is about 60-500 μm thick. According to yetanother embodiment, the bilayered material has an areal density of100-500 g/m². According to yet another embodiment, the bilayeredmaterial has an elastic modulus of 400 to 1,200 cN/dTex at anydirection. According to yet another embodiment, the bilayered materialhas an elongation modulus of 2-6% at any direction.

According to yet another embodiment, the method of this aspect furthercomprises:

-   -   (e) repeating steps (a) to (d) at least once, thereby obtaining        a plurality of bilayered materials; and    -   (f) compressing-molding at least two bilayered materials        obtained in step (e) thereby obtaining a multilayered material.

According to an alternative embodiment, step (e) is repeated until thedesired number of bilayered materials is obtained. According to yetanother embodiment, step (f) is conducted at a temperature below themelting point of the ultrahigh molecular weight polyethylene. Accordingto yet another embodiment, step (f) is conducted at a temperaturebetween 90 and 150° C. and, optionally, under a pressure of 100 to 200bar.

According to yet another embodiment, the multilayered material obtainedby the method of this aspect is a ballistic resistant article.

According to yet another embodiment, the method of this aspect furthercomprises the step of: cutting the multilayered material to a desiredshape.

According to yet another embodiment, the first unidirectionally orientedmonolayer provided in step (a) of the method of this aspect has atensile strength within the range of 10 to 25 cN/dTex. According to yetanother embodiment, the first unidirectionally oriented monolayerprovided in step (a) is about 30-120 μm thick. According to yet anotherembodiment, the first unidirectionally oriented monolayer provided instep (a) has an areal density of 60-200 g/m². According to yet anotherembodiment, the first unidirectionally oriented monolayer provided instep (a) has an elastic modulus of 400 to 1,200 cN/dTex. According toyet another embodiment, the first unidirectionally oriented monolayerprovided in step (a) has an elongation modulus of 2-6%.

According to yet another embodiment, each one of the unidirectionalultrahigh-molecular-weight polyethylene strip provided in step (b) ofthe method of this aspect has a tensile strength within the range of 8to 40 cN/dTex. According to yet another embodiment, each one of theunidirectional ultrahigh-molecular-weight polyethylene strip provided instep (b) is about 30-120 μm thick. According to yet another embodiment,each one of the unidirectional ultrahigh-molecular-weight polyethylenestrip provided in step (b) has an areal density of 60-200 g/m².According to yet another embodiment, each one of the unidirectionalultrahigh-molecular-weight polyethylene strip provided in step (b) hasan elastic modulus of 400 to 1,200 cN/dTex. According to yet anotherembodiment, each one of the unidirectional ultrahigh-molecular-weightpolyethylene strip provided in step (b) has an elongation modulus of2-6%.

Yet another preferred method relates to the preparation of an ballisticresistant article, which comprises:

-   -   (a) aligning a plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips such that each        strip is oriented in parallel to adjacent strips, wherein        adjacent strips partially overlap, said plurality of strips        being laid over a first unidirectional        ultrahigh-molecular-weight polyethylene monolayer, with the        direction of said plurality of strips being rotated with respect        to the direction of said first unidirectional        ultrahigh-molecular-weight polyethylene monolayer at an angle;        and    -   (b) compressing said plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips laid over said        unidirectional ultrahigh-molecular-weight polyethylene        monolayer, thereby obtaining a bilayered material consisting        essentially of ultrahigh-molecular-weight polyethylene and        essentially devoid of bonding matrices, comprising two        compressed unidirectional monolayers with the direction of each        monolayer being rotated at an angle with respect to the        direction in an adjacent unidirectional monolayer, with the        bilayered material being essentially devoid of resins or        adhesives.

If desired, the method can include the additional step of associatingthe bilayered material with an additional material selected from thegroup consisting of: ceramic, steel, aluminum, titanium, glass andgraphite to form a further ballistic resistant material.

Polyethylene Strips and Monolayers Comprising Same

A unidirectional polyethylene monolayer according to the presentinvention is composed of at least two high-strength unidirectionalpolyethylene strips. The strips partially overlap. In variousembodiment, the overlapping area is covered with a thin strip of lowdensity polyethylene prior to the compression and thereby formation ofthe unidirectional monolayer.

The terms “unidirectional films” or “unidirectional strips” areinterchangeably used herein to describe polyethylene films formed bycompression-molding and unidirectional stretching of polyethylene. Thedirection of stretching determines the direction of the resulting films.The unidirectional films can be split along the direction of stretching.

Homopolymers of polyethylene are particularly suitable for preparing theunidirectional strips. The high-molecular weight polyethylene may belinear or branched. High molecular weight here means a molecular weightof at least 400,000 g/mol.

The term “linear polyethylene” as used herein refers to polyethylenehaving fewer than 1 side chain per 100 carbon atoms, preferably fewerthan 1 side chain per 300 carbon atoms. The polyethylene may alsocontain up to 5 mol % of one or more other alkenes which arecopolymerisable therewith, such as propylene, butene, pentene,4-methylpentene, octene.

UHMW polyethylene films may be obtained using methods known in the art,for example, U.S. Pat. Nos. 4,879,076; 5,091,133; 5,106,555; 5,106,558and 5,200,129 among others.

According to one embodiment, a UHMW polyethylene film is prepared bydrawing particulate UHMW polyethylene powder at a temperature lower thanthe melting point thereof thereby obtaining a unidirectionally orientedUHMW polyethylene film exhibiting high tensile strength at the directionof stretching.

Drawing comprises the steps of: compression-molding, rolling andstretching. According to one embodiment, the polyethylene films areprepared as follow:

-   -   1. feeding a polyethylene in a powder form between a combination        of endless belts disposed in an up-and-down opposing relation;    -   2. compression-molding the polyethylene powder at a temperature        lower than the melting point of the polyethylene powder by        pressing means; and    -   3. rolling the resultant compression-molded polyethylene,        followed by stretching at a single direction, thereby obtaining        a unidirectional polyethylene film.

At the first step, of feeding, the polyethylene powder must bedistributed evenly between the belts. Uneven distribution may lead tothe formation of a film having a non-homogenous thickness that cannot beproperly stretched.

Pressing means may be employed by holding the polyethylene powderbetween the endless belts and conveying the same, said pressing meanscomprising pressing platens and corresponding sets of rollers, allaccommodated within the respective endless belts, the rollers in eachset being connected together, and said sets of rollers being arrangedmovably in an endless fashion between the respective platens and theendless belts associated therewith.

Stretching commonly comprises extrusion stretching and tensilestretching commonly employed in the art. To attain great mechanicalstrength and high elastic modulus, two-stage drawing is preferred inwhich particulate polyethylene is first extrusion-stretched, followed bytensile stretching of the extrudate.

Compression can preferably be conducted prior to solid phase extrusionor rolling. There is no particular restriction imposed on the method ofcompressing particulate polyethylene. In the case of solid phaseextrusion, the polymer may be compressed in the above extruder into arod-like shape at a temperature below its melting point and in a widerange of pressures. Particulate polyethylene and a different type ofpolymer may be pressed together into a monolayer of 30-120 μm thicknessat a temperature below their respective melting points and at a similarpressure. In the case of rolling, particulate polyethylene may becompressed by a suitable known method into a film or monolayer in whichinstance pressing is preferred as in co-extrusion.

According to one embodiment, the tensile strength of a 2 mm split cutfrom an ultrahigh molecular weight polyethylene strip of the inventionis 8 to 40 cN/dtex. All tensile properties are evaluated by methodsknown in the art, for example, by pulling a 10 in. (25.4 cm) splitlength clamped in barrel clamps at a rate of 10 inch/min (25.4 cm/min)on an Instron Tensile Tester.

The polyethylene strip according to the present invention is typically 5to 25 cm wide. Broader or narrower polyethylene films are similarlyuseful for the preparation of the ballistic resistant article of thepresent invention. The areal density of the polyethylene strip 60-200g/m². The elongation modulus of the polyethylene strip is within therange of 2-6%. According to an alternative embodiment, the maximaldisplacement of the polyethylene strip is 2-12 mm/200 mm.

Polyethylene Materials

The polyethylene monolayers of the present invention comprise aplurality of high-strength unidirectional polyethylene strips, theunidirectional polyethylene strips are oriented in parallel in oneplane, next to one another. According to some embodiments, the parallelpolyethylene strips partially overlap, the overlapping area is between 5μm to 40 mm wide. According to an alternative embodiment, the parallelpolyethylene strips contact each other but do not substantially overlap,wherein a narrow secondary polymeric film strip, about 5 to 20 μm wide,is laid over the axis of contact between two adjacent parallelpolyethylene strips. The parallel polyethylene strips, eitheroverlapping or substantially not overlapping, are then compressed undera temperature below the melting temperature of the polyethylene,preferable at 110 to 150° C. and under a pressure of 10 to 100 N/cm².The resulting polyethylene monolayer is unidirectional wherein thedirection of the monolayer is essentially the direction of the parallelpolyethylene strips therein.

The areal density of the polyethylene material consisting of a bilayerof two ultrahigh molecular weight polyethylene monolayers is 100-500g/m².

Ballistic Resistant Polyethylene Article

The present invention further related to ballistic resistant moldedarticles comprising unidirectional high-strength monolayers stackedcross-wise, the monolayers consisting essentially of ultrahigh molecularweight polyethylene and essentially devoid of resins and other bondingmaterials, wherein the stack is compressed at a given pressure andtemperature for a given time.

According to one embodiment, the present invention provides a ballisticresistant polyethylene article comprising 70 to 280 polyethylenemonolayers compressed at an angle to one another.

The National Institute of Justice (NIJ) rates body armor on ballisticprotection levels. As layers of a ballistic fiber (e.g DuPont KEVLAR®)or ballistic films being added, protection is enhanced. Vests are testednot just for stopping penetration, but also for blunt trauma protection,namely the blow experienced by the body as a result of ballistic impacton the vest. Blunt trauma is measured by the dent suffered by a softclay backstop to the vest, wherein a maximum of 1.7″ (44 mm) is allowed.

A bulletproof jacket comprising a multilayered article of the presentinvention, the multilayered article consisting of 200 to 300 monolayersand having an areal density of approximately 17 kg/m², provides a levelIII standard NIJ protection.

The ballistic resistant polyethylene article is also resistant tostabbing with knives or other sharp elements. For this purposeresistance may be achieved by using an article comprising a fewmonolayers.

According to various embodiments, the ballistic resistant polyethylenearticle is provided in combination with an additional material selectedfrom the group consisting of: ceramic, steel, aluminum, titanium, glassand graphite. The ballistic resistant polyethylene article and theadditional material may be bonded or otherwise retained by support meanssuch as fiberglass, fabric, polymeric plastics and elastomers and thelike.

Additionally, due to the exceptionally high heat resistance of thearticle of the invention, the article may be suitable as covering forrunning engines, for example, combustion engines.

The overall thickness requirements and weight of the ballistic resistantarticles of the present invention are greatly reduced as compared tosimilar ballistic resistant articles of the prior art.

The UHMW polyethylene article of the present invention may have anydesired dimension, depending upon the particular application thereof.The article may be flat or contoured and may have relatively largesurface area. For example, the article size may be such that it issuitable for covering the entire thorax region of a wearer.

Methods for Preparing Polyethylene Monolayers, Materials and BallisticResistant Articles

The present invention provides methods for preparing a unidirectionalmonolayer consisting essentially of ultrahigh molecular weightpolyethylene and essentially devoid of resins and any other bondingmatrices. The present invention further provides method for preparingrigid and impenetrable materials from unidirectional high-strengthpolyethylene monolayers and method for preparing ballistic resistantmultilayered articles from said polyethylene monolayers.

According to yet another aspect, the present invention provides a methodfor the continuous preparation of a unidirectional monolayer consistingessentially of ultrahigh-molecular-weight polyethylene, comprising:

-   -   (a) providing a plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips;    -   (b) aligning the plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips in the same        direction, wherein adjacent strips partially overlap; and    -   (c) compressing said plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips thereby obtaining        a unidirectional ultrahigh-molecular-weight polyethylene        monolayer.

According to one embodiment, compressing said plurality ofunidirectional ultrahigh-molecular-weight polyethylene strips occurs ata temperature below the melting point of the ultrahigh-molecular-weightpolyethylene. Typically, compressing said plurality of unidirectionalultrahigh-molecular-weight polyethylene strips occurs at a temperaturebetween 10 to 150° C. and a pressure of 10 to 100 N/cm².

According to a particularly preferred embodiment, the unidirectionalultrahigh-molecular-weight polyethylene monolayer essentially devoid ofbonding matrices.

According to yet another embodiment, the unidirectionalultrahigh-molecular-weight polyethylene monolayer has a tensile strengthwithin the range of 8-40 cN/dTex. According to yet another embodiment,the unidirectional ultrahigh-molecular-weight polyethylene monolayer hasan elastic modulus of 400 to 1,200 cN/dTex. According to yet anotherembodiment, the unidirectional ultrahigh-molecular-weight polyethylenemonolayer has a maximal displacement of 2-12 mm per 200 mm. According toyet another embodiment, the unidirectional ultrahigh-molecular-weightpolyethylene monolayer has an elongation modulus within the range of2-6%.

According to an alternative embodiment, the method comprising after step(b) and before step (c) the step of: laying a strip of polymeric filmover each area of contact between two adjacent unidirectionalultrahigh-molecular-weight polyethylene strips, the strip of polymericfilm is laid in parallel to the direction of the aligned plurality ofunidirectional ultrahigh-molecular-weight polyethylene strips.

According to one embodiment, the strip of polymeric film is 4 to 25 μmthick. The polymeric film may be selected from the group consisting of:low density polyethylene, high density polyethylene, polyamide,polypropylene, polyester, polystyrene, polyethylene, polycarbonate andpoly(methyl methacrylate).

According to yet another aspect, the present invention provides a methodfor the continuous preparation of a material comprising a plurality ofcompressed unidirectional monolayers consisting essentially ofultrahigh-molecular-weight polyethylene and essentially devoid ofbonding matrices, with the direction of each monolayer being rotatedwith respect to the direction in an adjacent unidirectional monolayer,the method comprising:

-   -   (a) providing a first unidirectional ultrahigh-molecular-weight        polyethylene monolayer;    -   (b) providing a plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips;    -   (c) laying the plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips over the first        unidirectional ultrahigh-molecular-weight polyethylene        monolayer, such that said plurality of strips are aligned in the        same direction wherein adjacent strips partially overlap,        wherein the direction of said plurality of strips being rotated        with respect to the direction of said first unidirectional        ultrahigh-molecular-weight polyethylene monolayer at an angle;        and    -   (d) compressing said plurality of unidirectional        ultrahigh-molecular-weight polyethylene strips laid over said        unidirectional ultrahigh-molecular-weight polyethylene        monolayer, thereby obtaining a bilayered material consisting        essentially of ultrahigh-molecular-weight polyethylene and        essentially devoid of bonding matrices, comprising two        compressed unidirectional monolayers with the direction of each        monolayer being rotated with respect to the direction in an        adjacent unidirectional monolayer at an angle.

According to certain embodiments, the plurality of strips are rotated atan between 20 to 160 degrees with respect to the direction of said firstunidirectional ultrahigh-molecular-weight polyethylene monolayer.According to other embodiments, the angle is of 70 to 110 degrees.According to yet other embodiments, the angle is of substantially 90degrees.

According to an alternative embodiment, the method further comprisesafter step (c) and before step (d) the step of laying a strip ofpolymeric film over each area of contact between two adjacentunidirectional ultrahigh-molecular-weight polyethylene strips, the stripof polymeric film is laid in parallel to the direction of the alignedplurality of unidirectional ultrahigh-molecular-weight polyethylenestrips.

According to one embodiment, the strip of polymeric film is 4 to 25 μmthick. According to another embodiment, the polymeric film is selectedfrom the group consisting of: low density polyethylene, high densitypolyethylene, polyamide, polypropylene, polyester, polystyrene,polyethylene, polycarbonate and poly(methyl methacrylate).

According to another embodiment, step (d) occurs at a temperature belowthe melting point of the ultrahigh-molecular-weight polyethylene,commonly between 100 to 150° C. and under a pressure of 10 to 100 N/cm².

According to one embodiment, the bilayered material obtained by themethod of this aspect has a tensile strength within the range of 8 to 40cN/dTex at any direction. According to yet another embodiment, thebilayered material is about 60-500 μm thick. According to yet anotherembodiment, the bilayered material has an areal density of 100-500 g/m².According to yet another embodiment, the bilayered material has anelastic modulus of 400 to 1,200 cN/dTex. According to yet anotherembodiment, the bilayered material has an elongation modulus of 2-6%.

Since the bilayered material comprises two unidirectional monolayerscompressed at an angle to one another, its strength is no longerdependent on the direction of each monolayer encompassed therein.Rather, the foregoing parameters can be obtained from measuring thetensile strength, elastic modulus and elongation modulus of thebilayered material at any direction. The extent of these parameter isnot necessarily limited to a particular direction of the bilayeredmaterial.

According to another embodiment, the method further comprises:

-   -   (e) repeating steps (a) to (d) at least once, thereby obtaining        a plurality of bilayers; and    -   (f) compression-molding the plurality of bilayers obtained in        step (e) thereby obtaining a multilayered material.

According to an alternative embodiment, step (e) is repeated until thedesired number of bilayered materials is obtained.

Compression-molding is intended to mean that the stack of materials issubjected to a given pressure for a particular compression time at acompression temperature below the softening or melting point of theultrahigh molecular weight polyethylene. The required compression timeand compression temperature depend on the kind of polyethylene and onthe thickness of the stack and can be readily determined by one skilledin the art.

In the process of the invention the stack may be made starting fromloose unidirectional monolayers. However, loose monolayers are difficultto handle in that they easily tear in the direction of stretching. It istherefore preferred to make the multilayered stack from consolidatedmonolayer materials containing from 2 to 8, preferably 2, monolayersthat are compressed cross-wise. Consolidated is intended to mean thatthe monolayers are firmly attached to one another. The resultingmaterials are then stacked and compressed to obtain a multilayeredmaterial.

According to one embodiment, step (f) is conducted at a temperaturebetween 90 and 150° C. and under a pressure of 100 to 160 bar.

It has been found that in order for a high ballistic resistance to beattained it is necessary that after compression at a high temperaturecooling, too, take place under pressure. Cooling under pressure isintended to mean that the given minimum pressure is maintained duringcooling at least until so low a temperature is reached that thestructure of the multilayered article can no longer relax underatmospheric pressure so that the ballistic resistance value cannotdecrease. This temperature can be established by one skilled in the art.It is preferred for cooling at the given minimum pressure to be down toa temperature below the relaxation temperature of the polyethylenemonolayers of materials. The pressure during the cooling does not needto be equal to the pressure at the high temperature, but it is preferredthat these pressures are equal. The pressure loss resulting fromshrinkage of the molded multilayered article and the press due tocooling must regularly be compensated for so as to keep the pressureduring cooling constant or at least on a sufficient high level.

For the manufacture of a ballistic-resistant molded article in whicheach monolayer consists essentially of high-molecular-weightpolyethylene, the compression temperature is 90 to 150° C., preferably115 to 130° C. and, optionally, cooling to below 70° C. is effected at aconstant pressure. Compression temperature here means the temperature athalf the thickness of the molded article. The compression pressure is100 to 180 bar, preferably 120 to 160 bar and the compression time isbetween 40 to 180 minutes.

The following examples are to be construed in a non-limitative fashionand are intended merely to be illustrative of the principles of theinvention disclosed.

EXAMPLES Example 1 Solid of Ultrahigh Polyethylene

Various ultrahigh molecular weight polyethylene solid products can beused to form the strips and monolayers of the invention. For example,MIPELON™ (Mitsui Chemicals America, Inc.), HI-ZEX® MILLION (MitsuiChemicals America, Inc.) and PE type 1900CM (Basell USA Inc.), amongothers. The solid product is preferably a fine powder of ultra-highmolecular weight polyethylene, with a molecular weight of about 2·10⁵ to6·10⁶ microns or more and particle measurements of no more than 25 to200 microns.

Example 2 Preparation of a Polyethylene Film

A unidirectional polyethylene film was prepared from a fine powder ofUHMW polyethylene having the following properties: an average molecularweight range between 4 and 5 million, intrinsic viscosity—26 dl/g,density (molded part)—0.93 g/cm³, tensile stress at yield—2,700 psi (19Mpa), tensile elongation at yield—6%, tensile stress at break 6,000 psi(40 Mpa), tensile elongation at break 340% and bulk density—0.43-0.47g/cm³.

Rolling and Tensile Stretching: The polymer was pressed at 125° C. andat 0.02 GPa into a 0.2 mm thick film which was then passed at 130° C.through a pair of counter-rotating rollers each dimensioned to be 100 mmin diameter and 500 mm in crosswise length and having differentperipheral speeds, thereby forming a film drawn at a ratio of 6.

Stretch moldability of the resulting film was tested with a tensiletester and under conditions of temperature 120° C. and crosshead speed40 mm/min. Molding was possible at a draw ratio of 20.

The UHMW polyethylene film (FIG. 1) showed a melting point (peaktemperature) of 141° C. as measured without heat treatment bydifferential scanning calorimetry with temperature rise 5° C./min(DSC-20 calorimeter, manufactured by Seiko Denshi Kogyo K. K.).

Example 3 Preparation of an UHMW Polyethylene Monolayer

Unidirectional polyethylene strips (films), each strip is about 2-50 cm,typically 8 cm wide, are aligned next to one another in parallel, thedirection of alignment is parallel to the direction of the strips(namely, the direction of stretching) whereas the strips partiallyoverlap, the overlapping area is about 5 mm wide (6% of the width ofeach strip overlaps with 6% of the width of an adjacent strip). Theoverlapping strips are laminated at 140° C. under a pressure of 20N/cm². No resins or matrices are added. The density of the resultingmonolayer (FIG. 2) is ca. 62.5 g/m² and the thickness is about 6.7 μm.

Example 4 Preparation of an UHMW Polyethylene Article

Approximately 25 strips of a unidirectional polyethylene film are placedover a unidirectional polyethylene monolayer. Each strip is about 8 cmwide. The strips are aligned at the direction of stretching next to oneanother in parallel and in an angle of about 90 degrees to the directionof the monolayer (FIG. 3). The strips and the monolayer are compressedat 140° C. under a pressure of 20 N/cm². No resins or elastomericmatrices are added. As a result a two-plies UHMW polyethylene materialcomprising two UHMW polyethylene monolayers devoid of resins and otherbonding matrices is obtained.

The process is repeated until a desired number of two-plied UHMWpolyethylene materials is obtained. The two-plied UHMW polyethylenematerials are laid one on top of the other to form a stack. The stack iscut to a desired shape and then pressed at a temperature of about 120°C. and under a pressure of 140 bar for about one hour. The resultingarticle is present in FIG. 4.

Example 5 Ballistic Resistance of an UHMW Polyethylene Article

The antiballistic properties of the article of the invention were testedusing a multilayered article consisting essentially of ultrahighmolecular weight polyethylene manufactured according to the methods ofthe invention. The tested multilayered article had an areal density of20.85 kg/m² and a size of 250×300 mm². A backing comprising 30 layers ofaramid fibers (Twaron™, Akzo Nobel) was laid on the multilayeredarticle, without any bonding materials. The backing and the multilayeredarticle were positioned in a fabric pocket together with a 102 mm claybacking.

The ballistic test was carried out with a Barrel gun, 24 inch barrellength and a caliber of 7.620 mm, bullet type M-80 and bullet weight of148 grain. Angle of shot was 0 degrees. The results of the ballistictest are summarized in Table 1.

TABLE 1 TRAUMA depth/ width VELOCITY PENETRATE Shot No. (mm) m/secft/sec (Y/N) 1 37 0 855 2805 N 2 38 0 856 2810 N 3 0 0 852 2796 N 4 0 0849 2784 N 5 0 0 854 2801 N 6 0 0 852 2796 N average of 12.5 0.0 8532798 not penetrating shots

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without undue experimentation and withoutdeparting from the generic concept, and, therefore, such adaptations andmodifications should and are intended to be comprehended within themeaning and range of equivalents of the disclosed embodiments. It is tobe understood that the phraseology or terminology employed herein is forthe purpose of description and not of limitation. The means, materials,and steps for carrying out various disclosed functions may take avariety of alternative forms without departing from the invention.

The invention claimed is:
 1. A multilayered material comprising: aplurality of monolayers compression-laminated to one another such thatthe film strips in adjacent monolayers are positioned at an angle withrespect to one another, wherein each of the monolayers comprises: (i) aplurality of polyethylene film strips oriented in parallel in one planenext to one another, the polyethylene film strips being elongated bodieshaving a length, a width and a thickness with the length and width beinglarger than the thickness, wherein the width of each polyethylene filmstrip is about 2 to 50 cm, and (ii) a polymeric film positioned overareas of contact between adjacent ones of the polyethylene film strips,wherein each of the polyethylene film strips is formed ofcompression-molded particulate consisting of ultra-high molecular weightpolyethylene and is uniaxially drawn; and wherein each of the monolayershas a thickness of 30 to 120 μm; and wherein the polyethylene filmstrips are arranged in a non-overlapping configuration; and wherein eachof the monolayers is devoid of bonding matrices.
 2. The multilayeredmaterial of claim 1, wherein the multilayered material consists of twomonolayers.
 3. The multilayered material of claim 1, wherein thepolyethylene film strips in one of the monolayers are positioned at anangle between 20 to 160 degrees with respect to the polyethylene filmstrips in an adjacent one of the monolayers.
 4. The multilayeredmaterial of claim 1, wherein the monolayers have a tensile strengthwithin the range of 8-40 cN/dTex.
 5. The multilayered material of claim1, wherein the monolayers have an areal density of 60 to 200 g/m². 6.The multilayered material of claim 1, wherein the monolayers have anelongation modulus of 2-6%.
 7. The multilayered material of claim 1,wherein the monolayers have an elastic modulus of 400 to 1,200 cN/dTex.8. The multilayered material of claim 1, wherein the monolayers have amaximal displacement of 2-12 mm per 200 mm.
 9. The material of claim 1,wherein the width of each polyethylene film strip is about 5 to 25 cm.10. The material of claim 1, wherein the aspect ratio of width tothickness of each polyethylene film strip is at least 1000:6.
 11. Amultilayered material comprising: a plurality of monolayerscompression-laminated to one another such that the film strips inadjacent monolayers are positioned at an angle with respect to oneanother, wherein each of the monolayers comprises: (i) a plurality ofpolyethylene film strips oriented in parallel in one plane next to oneanother, and (ii) secondary strips of polymeric film positioned over anarea of contact between adjacent ones of the polyethylene film strips,wherein the polyethylene film strips are elongated bodies formed ofcompression-molded particulate consisting of ultra-high molecular weightpolyethylene and are uniaxially drawn, the film strips having a length,a width and a thickness with the length and the width being larger thanthe thickness, and wherein the polyethylene film strips are arranged ina non-overlapping configuration; and wherein each of the monolayers isdevoid of bonding matrices.
 12. The multilayered material of claim 11,wherein the secondary strips of polymeric film have a width of about 5to 20 μm.
 13. The multilayered material of claim 11, wherein thesecondary strips of polymeric film have a thickness of about 4 to 25 μm.14. An article comprising the multilayered material of claim 1 incombination with an additional material selected from the groupconsisting of ceramic, steel aluminum, titanium, glass and graphite. 15.A ballistic resistant article comprising the multilayer material ofclaim
 1. 16. The ballistic resistant article of claim 15, wherein theballistic resistant article comprises between 70 to 280 monolayers, andwherein the ballistic resistant article optionally comprises anadditional material selected from the group consisting of ceramic,steel, aluminum, titanium, glass and graphite.
 17. A fabric envelopecomprising the ballistic resistant article of claim
 15. 18. A supportingstructure for being worn on a body part, wherein the supportingstructure comprises the ballistic resistant article of claim
 15. 19. Thesupporting structure of claim 18 in the form of a body garment.